Conventionally, as a nonaqueous electrolyte secondary battery, which is a type of energy storage device, one described in Patent Document 1 is known. This nonaqueous electrolyte secondary battery includes a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode. The negative electrode includes a negative composite layer formed on a surface of a negative electrode current collector made of a metal. In addition, the separator includes a base material layer containing a thermoplastic resin as a main component and an inorganic layer containing a filler as a main component.
Patent Document 1 states the following. “The lithium secondary battery of the present invention includes a positive electrode, a negative electrode, a nonaqueous electrolyte, and a separator enclosed in a hollow columnar battery case and has the following characteristics. The positive electrode includes a positive composite layer containing a positive active material, an electrically conductive auxiliary, and a binder on one or both sides of a current collector. As the positive active material, a lithium-containing composite oxide containing lithium and a transition metal is used. At least part of the lithium-containing composite oxide is a lithium-containing composite oxide containing nickel as a transition metal, and the molar ratio of the total nickel amount relative to the total lithium amount in the entire positive active material is 0.05 to 1.0. The separator includes a porous membrane (I) containing a thermoplastic resin as a main component and a porous layer (II) containing a filler having a heatproof temperature of 150° C. or higher as a main component. The side surface portion of the battery case includes two large-width surfaces that are opposed to each other and each have a larger width than other surfaces in side view. The side surface portion has formed therein a splitting groove that splits in the case where the pressure in the battery case exceeds the threshold. The splitting groove is provided so as to intersect a diagonal line from the large-width surfaces in side view” (paragraph 0009). As a result, “a lithium secondary battery having high capacity together with excellent safety at extremely high temperatures can be provided” (paragraph 0010).